What I did was as follows.
Firstly, I made a bread dough. It was a fairly plain one, basically white bread flour with yeast, salt, water and some olive oil in the mix.
Once risen, I rolled some of it out flat (the rest made a separate loaf), and lined a loaf tin with it.
Meanwhile, I'd made a white sauce to which I'd added parsley and tarragon, before also adding some pre-cooked mussels and also some sweetcorn. This was cooked to the point that it was thick rather than runny.
I added this to the loaf tin, and then covered that with a lid, also of rolled out dough. I pierced the lid a few times to avoid any explosions, and then put the whole thing in the hottest oven I can get for 35 minutes. (I also had a pyrex jug of water in there, to crisp up the crust a bit.)
The result was really rather yummy, having produced pretty much exactly what I'd hoped.